The present invention relates to the measuring of optical wavelengths and more specially to an interferometer apparatus for measuring the wavelength of laser radiation.
At the present state of the art, it is no trivial problem to measure the wavelength of optical radiation, such as laser radiation, with an accuracy of over 10.sup.6. An overview of shortcomings and pitfalls in this field and the newest measuring processes is comprised in the book "Laser Spectroscopy III. Proceedings of the Third International Conference, Jackson Lake Lodge, Wyoming, U.S.A., July 4-8, 1977" by J. L. Hall and J. L. Carlsten, published by Springer-Verlag Berlin-Heidelberg-New York 1977, chapter 9: Laser Wavelength Measurements, pages 410 to 426. The best and most widely used way of measuring laser wavelengths was firstly proposed by J. L. Hall and S. A. Lee (Applied Physics Letters 29, no. 6, Sept. 15, 1976, pages 367 to 369) and is based on the use of a Michelson-interferometer with moving mirrors. In this case, a reference beam whose wavelength has been measured with great accuracy and produced by a stabilized laser (for example an iodine stabilized He-Ne-laser) goes to a photoelectric detector and, somewhat to the side, but nevertheless parallel to the reference beam, a "test" or sample beam (that is to say one whose wavelength is to be measured) goes to a second detector. The interferometer has a beam-splitter by which each of the incoming beams is divided into first and second part-beams, and the length of the optical path travelled by the first part-beams can be varied in relation to the length of the optical path travelled by the other part-beam by movable mirrors. When the mirrors are moved, the output beams from the interferometer are intensity-modulated by the occuring interference so that the two detectors produce electrical output signals with a similar modulation. The modulation frequency of the reference beam and of the test beam is measured, e.g. by counting, and the relation between the wavelengths of the reference and test beam can be calculated from the values obtained. To obtain an accuracy of over 10.sup.6, the moving mirrors of this known apparatus have to be shifted in position by about 1 m, this needing about 25 seconds. Thus, the known apparatus takes up much space and, because of its great size, great care must be taken to secure a vibration-free support. The apparatus can only be used at the place of its permanent installation and the radiation sources, whose wavelength is to be measured, have to be taken to it.